Which letter/biography? Is this in translation or original? What was the sentence/paragraph before? etc etc. Just asking for context (because this is definitely not a common English term.
– MitchDec 15 '18 at 21:09

The spelling 'Dedjatch' is presumably a francophone spelling ('dj' is how the French spell the rare in French voiced palatal affricate which is 'j' in English).

Searching for 'dejatch' yields many sources saying "Prince (Dedatch) Alemayehu" or just "Prince Alemayehu", and the interpretation that 'Dejatch' is a title of nobility works in all the contexts it is mentioned in those texts.

The usual term for 'prince' in Amharic is ልዑል= liuli, the son of the emperor. But translation and cultural choices can be arbitrary and the English (and French and Italian) somehow settled on 'prince' in this situation.